The present invention relates to electronic devices and more particularly to packaging for electronic devices.
Semiconductor electronic devices often include a package surrounding the device to permit handling by a user such that only a very isolated portion of the device is exposed to external environmental influences and usually this exposed portion is only the contact points for electrical coupling including: input, output, biasing, and signaling connections. Packages are typically manufactured out of plastic, metal, ceramic or glass.
For circuits that operate in the gigahertz and above frequency range, the package has additional constraints including that the package must be designed so as not to cause performance limitations. If not constrained, electric signals that operate at microwave frequencies are directed as waves in all directions causing a loss in the signal which is, proportional to the wavelength of the signal. Additionally, these waves can feedback into the signal path causing distortion to the signal. Without careful design, high return losses occur and therefore special consideration must be paid toward the propagation of the signal in the package.
A surface mount package for a microwave circuit is disclosed. The package has a port for transferring a signal from the microwave circuit to a printed circuit board. The package includes a base made from a dielectric material. Conductive material is placed on the topside of the base forming a transmission line for carrying the signal in a transmission line mode. To facilitate operation in a transmission line mode, the length of the transmission line is a substantial portion of the wavelength of the signal. The transmission line provides electrical contact with a terminal of the microwave circuit and is electrically coupled to the port, wherein the port partially defines the bottom side of the package. Additionally, the transmission line forms an electromagnetic wave guiding structure. In one embodiment, the electromagnetic wave guiding structure operates in grounded coplanar waveguide mode. In one alternative embodiment, the electromagnetic wave guiding structure operates in microstrip mode. In another embodiment, the electromagnetic wave guiding structure operates in both grounded coplanar waveguide and microstrip mode where a second conductive material resides on the top side of the base and spaced a distance from the first conductive material. In such a configuration, a portion of the base separates the first and second conductive materials, and the second conductive material substantially surrounds the first conductive material of the transmission line.
In one embodiment, the second conductive material is displaced from a first portion of the transmission line by a first distance so that the transmission line operates in grounded coplanar waveguide mode and by a second distance along a second portion of the transmission line so that the transmission line operates in microstrip mode along the second portion.
Additionally, there are a plurality of vias in the base. Each via is filled with a conductive material and the vias both direct the signal from the package to the microwave circuit and block propagation of the signal to prevent feedback of the signal to the microwave circuit. In one embodiment, the vias are all electrically coupled to a common ground. In such an embodiment, there are a sufficient number of vias to provide a substantial grounding of the base so that the signal does not feedback into the signal path. In another embodiment, the vias are spaced less than a quarter wavelength apart.
Another element to the package is a cover which resides on the base and encloses the microwave circuit. The microwave circuit is enclosed in such a way that the signal in its electromagnetic form is allowed to propagate in a controlled manner within the package. The cover is preferably made from a material which shares a similar thermal expansion coefficient to the base.
In an alternative embodiment, at least one bonding pad is provided for electrically coupling at least one terminal of the microwave circuit. In an embodiment in which there are a plurality of bonding pads, the bonding pads provide contact points for DC biasing the microwave circuit.
In yet another embodiment the base includes a conductively coated output castellation formed on the side of the base and the castellation defines the port of the package. In yet another variation, the conductive material residing in the output castellation electrically couples the transmission line to the printed circuit board.